This invention relates to an optical head device effective in reading or recording the signal from or onto various types of optical recording mediums (including a digital video disk (DVD) and a compact disk (CD)).
In the field of recording mediums, DVD has recently been developed which enables high-density recording, although having the same diameter (12 cm) as that of a conventional CD on which audio or digital data is recorded. Since having the high recording density, DVD requires a beam of light of a shorter wavelength (650 nm) than the wavelength (780 nm) of a beam of light used to read the data on the CD.
It is desirable that a reproducing apparatus should be capable of playing back a disk system of both the CD type and the DVD type. Thus, a disk system with a CD light source (with a wavelength of 780 nm) and a DVD light source (with a wavelength of 650 nm) has been developed as an optical head device (or an optical pickup) using a multi-wavelength semiconductor array.
The optical head device includes a first light source and a second light source and further a diffraction grating to which the light from either of the first or second light source is directed. The diffraction grating particularly separates a CD light beam into three beams. The beams of light emitted from the diffraction grating pass through a hologram and then through a collimator lens and turn into collimated light. The collimated light enters an objective lens. The light gathered by the objective lens is projected onto the recording surface of a disk. The light reflected from the disk passes through the objective lens and the collimator lens and enters the hologram. The hologram diffracts the reflected light and directs it to a photodetector composed of photodiodes. The photodetector outputs a focus error signal, a tracking error signal, and a read signal.
(A1) In the optical head device, when the device is set in the CD reading state, the laser beam with a wavelength of 780 nm from the light source is inputted to the diffraction grating, which then outputs three beams. One of the three beams is the main beam and the remaining two are sub-beams. The intensity ratio of the main beam to the sub-beam is about 8:1.
When the device is set in the DVD reading state, the laser beam with a wavelength of 650 nm from the light source also passes through the diffraction grating. This permits unnecessary side beams to occur. As a result, the efficiency of the main beam decreases. Naturally, in the photodetector receiving the reflected light from the disk, the light-receiving efficiency of the main beam decreases. This causes the problem of degrading the C/N of the signal. To improve the C/N of the signal, the amount of emission of a DVD light source may be increased. With this approach, however, there is a strong possibility that the device will be used beyond the maximum rating of the semiconductor laser, impairing the reliability of the device.
(B1) A conventional equivalent has disadvantages in that its overall mechanism is complex and its structure is great.
(C1) In the optical head device that reads both of CD and DVD signals, birefringence components might take place in the reflected light from a CD. In that case, the amount of light received by the photodetector fluctuates. The large amount of birefringence components causes a problem: the playback signal deteriorates seriously.
(D1) In the optical head device, when a first light source for DVD and a second light source for CD are used, the quality of the playback signal tends to deteriorate.
(E1) In addition, there is another problem: the level of the focus error signal in DVD playback drops.
(F1) There is still another problem: the quality of the focus error signal deteriorates.
(G1) To play back either a CD or DVD, a good playback performance is needed. To satisfy this requirement, a method of aligning the position of each component part must be improved.